In the course of practicing a wide variety of commercially important industrial processes, gaseous process streams (or more generally “gaseous systems”) are produced which are contaminated with pernicious quantities of mercury. The mercury contaminants have proved to be particularly difficult to remove or reduce to acceptable levels. One of the most harmful forms of mercury pollution is finely aerosolized elemental mercury. This form of mercury is generated by coal-fired power plants and also is present in natural gas. In the U.S. coal-fired power plants are the largest source of man-made mercury emissions to the air, accounting for approximately 40% of all mercury emissions. Under current circumstances, mercury is adsorbed on the aerosolized soot from coal burning. This soot eventually settles and the mercury adsorbed on the carbon is converted to methyl mercury, dimethyl mercury, and other forms, which accumulate in the food chain. Alternatively, techniques have been developed which will cause the carbonaceous soot to auto-ignite and convert to CO2 and H2O. When this occurs, finely aerosolized elemental mercury is produced. The mechanism for conversion of elemental mercury to methyl mercury and other forms is not well understood but is most certainly microbially mediated. It is estimated that 2000 tons of mercury is generated this way annually. Elemental mercury also occurs in natural gas in concentrations up to hundreds of micrograms per Nm3. This is a significant account considering that a typical plant will process millions of Nm3 per day.
Currently there is no technology that is considered optimal for remediation of the mercury in its elemental aerosolized form. Although coalescers, brominated adsorbents, and other methods have been used, they either lack effectiveness or have significant negative aspects such as generation of large amounts of mercury-polluted material to be land-filled. Coalescers lack effectiveness due to the extremely small size and high surface tension of the droplets and also due to the lack of affinity for mercury of typical coalescer materials. Also known is a process based on photochemical oxidation. This has chiefly been known for use in treating flue gas wherein ultraviolet (UV) light is introduced into the flue gas, to convert elemental mercury to an oxidized form (i.e. mercuric oxide, mercurous sulfate, and mercurous chloride). Once in the oxidized form, mercury can be collected in existing air pollution control devices such as wet SO2 scrubbers, electrostatic precipitators, and baghouses (fabric filters).
None of the foregoing techniques, however, have been fully successful in treating gaseous systems of the type with which the present invention is concerned. In addition to human and ecological effects, mercury in this elemental finely aerosolized form compromises the integrity of the steel and iron in the plants and pipeline for processing and transporting the gas, sometimes resulting in catastrophic failure and explosions or uncontrolled releases. It would be most desirable to capture and coalesce the droplets of mercury and to remove the mercury from these gaseous streams in its pure and elemental form, thus eliminating release and/or production of great quantities of mercury-polluted adsorbent.
The aforementioned problems in treating gaseous streams are also issues in aqueous or gaseous process streams (or more generally “fluid streams”) which are produced in many other commercially important industrial processes, these streams being contaminated with pernicious quantities of mercury in various oxidation and complexation states and including elemental, ionic, and organically-bound mercury. Mercury is corrosive to metals and other materials within a facility where the process is practiced, and is harmful to human health and to the surrounding ecosystem. Mercury contaminants have proved to be particularly difficult to remove or reduce to acceptable levels. In order to do so, it is important to know the concentration and speciation (organically-bound, ionic, or elemental) of mercury (Hg) in the stream containing same.
The parent application to the present disclosure, U.S. patent application Ser. No. 13/392,357, discloses methods and apparatus for analyzing the concentrations of diverse contaminating mercury species present in a fluid stream, whether aqueous or gaseous, in order that an effective strategy for separating the mercury from the stream may then be formulated. Characterization of the particular mercury (Hg) species in a waste stream is important in designing remediation technology, as the three primary forms of mercury (ionic, organically-bound, and elemental) possess very different physical and chemical properties. However, up to now, the ability to characterize mercuric species has been limited and difficult. Mercury is usually present in very low concentrations (usually 1 ppm or less) and there are usually large fluctuations in influent mercury concentration; rendering inaccurate spot sampling. The composition of speciation changes when these small amounts of mercury come in contact with the sample vessel. Further, standard tests are destructive and do not differentiate adequately between the three forms. The parent application U.S. patent application Ser. No. 13/392,357 disclosed a multi-stage filtration method and system to address the problems in prior systems.